1. Field of the Invention
The present invention relates to a process for forming color images and particularly to a process for forming color images suitable for producing multicolor optical filters for color camera tubes or color solid-state camera devices.
2. Description of the Prior Art
In color tubes and color solid-state camera devices such as a charge coupled device (CCD), a charge injection device (CID) or a bucket brigade device (BBD), a multicolor striped or mosaic optical filter is used. Generally, in the multicolor optical filter, three colors comprising red, green and blue, or cyan, magenta or yellow are systematically arranged, but their color constitution is not always limited to these three colors and it sometimes comprises two colors or 4 or more colors.
Particularly, in the color optical filter used for the color solid-state camera devices, the filter must be placed as close to the surface of the solid-state camera device as possible in order to prevent color stain which occurs when light passing through the filter falls on an adjacent color element. A process for producing a multicolor optical filter which satisfies the above described requirements has been proposed in U.S. Pat. No. 4,081,277. This process comprises applying a cross-linkable gelatin photoresist to a dye accepting polymer layer through which the dye is capable of thermally diffusing, developing the photoresist layer by a conventional process to form a resist pattern, covering this pattern with a dye-containing binder layer such that the dye thermally diffuses into the exposed dye accepting layer, and removing the resist pattern to obtain a color image. This process is carried out repeatedly to form in turn color images each having a different color at a different position by which a multilayer color optical filter is produced. According to this process, since the multicolor filter is produced in one dye accepting polymer layer, it is possible to dispose the filter in close contact with the surface of the solid-state camera device and, consequently, light passing through the filter does not produce color stain in the formed image, because the light does not impinge on adjacent color elements in the solid-state camera device. Further, it is possible to attain a very high resolving power since the light hitting the solid-state camera device is correctly focused.
However, this process has the following shortcoming, a relief pattern composed of a gelatin photoresist is used as a mask in thermally diffusing the dye into the uncovered dye accepting layer. Because the gelatin photoresist layer is produced from a polymer similar to the dye accepting layer, the dye diffuses into not only the dye accepting layer but also into the relief pattern itself during thermal diffusion. If the gelatin resist layer is too thin, the dye diffuses into the dye accepting layer below the relief and causes fogging. Such fogging causes color stain in the case of a multicolor optical filter. If the thickness of the resist is increased in order to overcome this fault, diffusion of light occurs in the resist layer when it is imagewise exposed to light and it is impossible to obtain images having a high resolving power. Particularly, in case of carrying out exposure by closely contacting an optical mask in order to obtain fine striped or small mosaic images as in case of the multicolor optical filter, images having the desired size can not be obtained, because the light is diverted behind the mask image.
Further, if it is attempted to thermally diffuse a second dye and a third dye into the dye accepting polymer containing the first dye image formed previously in order to obtain a multicolor optical filter using the above described process, the dyes previously thermally diffused into the dye accepting polymer thermally diffuse into other materials during the thermal diffusion step to destroy the dye image and deteriorate the sharpness of the edges of the image.
On the other hand, the process described in British Pat. No. 1,353,063 has been known as one of those recording processes using light having a high energy density such as a laser, etc. This process is a marking process which comprises supplying fine particles such as a pigment, etc. to the surface of an element having a thermofusible surface so as to form a thin layer, applying a laser beam to the fine particle layer to fuse the thermofusible layer by which the fine particles in the fused areas are fixed to the thermofusible layer. The fine particles which are not fixed are then removed from the surface.
This process is effective for scribing the surface of, for example, a steel plate, etc. However, this process suffers the following drawback, when it is used to obtain, a multicolor optical filter. Namely, since the color image obtained by this process is formed by fusing the thermofusible surface to which the laser beam is applied, light passing through this color image is scattered in the filter and, consequently, in case of, particularly, a filter for a color solid-state camera device, the light passing through the filter hits adjacent color elements in the solid-state camera device and causes color stain on the formed image. Further, since the light transmittance is naturally reduced in the fused areas, it becomes impossible to produce a preferred filter.